Conventional propellant-based downhole stimulation tools typically employ a right circular cylinder of a single type of propellant, which may comprise a single volume or a plurality of propellant “sticks” in an outer housing. Upon deploying such a downhole stimulation tool into a wellbore adjacent a producing formation, a detonation cord extending through an axially-extending hole in the propellant grain is typically initiated and high pressure gases generated from the combusting propellant grain exit the outer housing at select locations, entering the producing formation. The high pressure gases may be employed to fracture the producing formation, to perforate the producing formation (e.g., when spatially directed through apertures in the housing against the wellbore wall), and/or to clean existing fractures formed in the producing formation by other techniques, any of the foregoing increasing the effective surface area of the producing formation available for production of hydrocarbons.
U.S. Pat. Nos. 7,565,930, 7,950,457 and 8,186,435 to Seekford, the disclosure of each of which is hereby incorporated herein in its entirety by this reference, propose a technique to alter an initial surface area for propellant burning, but this technique cannot provide a full regime of potentially available ballistics for propellant-induced stimulation in a downhole environment. It would be desirable to provide enhanced control of not only the initial surface area (which alters the initial rise rate of the gas pulse, or dP/dt, responsive to propellant ignition), but also the duration and shape of the remainder of the pressure pulse introduced by the burning propellant.
U.S. patent application Ser. No. 13/781,217 by the inventors herein, filed Feb. 28, 2013, now U.S. Pat. No. 9,447,672, issued Sep. 20, 2016, and assigned to the Assignee of the present disclosure, addresses many of the issues noted above and left untouched by Seekford.
Unfortunately, the configurations of conventional propellant-based downhole stimulation tools offer limited to no means of controllably varying the pressure within a producing formation over an extended period of time (e.g., a period of time greater than or equal to about 1 second, such as greater than or equal to about 5 seconds, greater than or equal to about 10 seconds, greater than or equal to about 20 seconds, or greater than or equal to about 60 seconds).
It would, therefore, be desirable to have new downhole stimulation tools and methods of stimulating a producing formation, which facilitate controllably varying the pressure within the producing formation over an extended period of time. In addition, it would be desirable if the downhole stimulation tools and components thereof were easy to fabricate and assemble, exhibited nominal movement within a wellbore during use and operation, and were at least partially reusable.